The invention relates to a hearing aid with a microphone, a transmission section for signal processing and an output amplifier with connected earphone.
Output amplifiers for hearing aids should have a low power consumption, even for a high output power, in addition to low distortion.
Class B amplifiers possess a higher efficiency than Class A amplifiers. Amplifiers of this type have also been usual for hearing aids in the past.
Output amplifiers in the form of switching amplifiers have an even better efficiency, since the losses in the switches can be theoretically zero.
Known switching amplifiers use pulse duration modulation.
Examples of such D amplifiers are disclosed and described in detail in the European patent application 0 590 903 A1 of Exar Corporation and in U.S. Pat. No. 5,247,581 of Exar Corporation as well as U.S. Pat. No. 4,689,819 and U.S. Pat. No. 4,592,087 of Industrial Research Products Inc., for example.
Such D amplifiers operate in principle as follows:
The square-wave pulse sequence of an oscillator in the ultrasonic frequency range is supplied to an integrator, to which the output voltage of a low-frequency signal is also supplied, whereby this signal arrives from a microphone via an amplifier arrangement and serves as a bias voltage. The output signal of the integrator is then a sawtooth pulse sequence, of which the zero crossings can be varied by the bias voltage in the audible frequency range that is supplied to the integrator. In other words, this low-frequency bias voltage variably shifts the zero crossings of the sawtooth signal from a characteristic which is symmetrical to the symmetry axis without bias voltage signal to an asymmetrical state, whereby the sign and magnitude of the asymmetry are a continuously changing function of the amplitude of the low-frequency input signal.
The zero crossings are then used to control the timing and polarity of the output signal of a polarity-reversing, symmetrical CMOS switching driver stage, which varies the duration of the positive and negative switching pulses corresponding to the time displacement between the zero crossings of the integrator output signal, and thus transmits a pulse-modulated output signal to the earphone with a frequency spectrum in the low-frequency range and which represents an amplified image of the output signal of the microphone.
Such D amplifiers operating with pulse duration modulation have a very high efficiency and operate with hardly any crossmodulation.
A disadvantage of D amplifiers with pulse duration modulation is that the pulse duration should be changed either continuously or in quite small steps if it is wished to achieve a high signal-to-noise ratio.
The known Class D output amplifiers use continuous modulation, i.e. continuous variation of the pulse duration, and therefore need a continuous output signal of the microphone as an input signal. If signal processing preceding the output amplifier takes place discretely with respect to time or amplitude, then this digital signal must first be converted, e.g. in a holding network or a digital-to-analog converter. This represents a hardly justifiable additional measure.
A sigma-delta converter is known from EP-A 0495328, for example, which is particularly suitable as an A/D converter with discrete components. Such circuits are, however, less suitable for use in hearing aids with highly integrated digital circuits.
In addition, EP-A0597523 discloses a fast D/A converter consisting of a sigma-delta converter and of a downstream asynchronous sigma-delta modulator which generates an ambivalent, asynchronously modulated signal from the output signal of the sigma-delta converter, said modulated signal being forwarded to a low-pass filter.
Here also, the level of complexity for the output amplifier of a fully digitized hearing aid is far too high. In addition, it is not possible to achieve a high signal-to-noise ratio.
A hearing aid is known from WO 89/04583 that consists of a part to be worn on the ear and a signal processing part connected via a cable to be worn on the body, in which digital signal processing is performed by means of an AID converter and fitting of the transmission function of the hearing aid to the hearing impairment of the wearer is realized by means of a downstream D/A converter.
The level of complexity here, particularly due to the use of an AID converter, a signal processor and a downstream D/A converter, is far too high and is unsuitable for a fully digitized hearing aid. In addition, an extremely high signal-to-noise ratio cannot be achieved with such a circuit.
Finally, EP-A 0578021 discloses a hearing aid that does not contain a sigma-delta converter but a normal AID converter, a signal processing circuit and a D/A converter.
These circuit parts are followed by a modulator which generates a PDM signal that has to be then forwarded to a low-pass filter. Here too, the level of complexity is too high, in addition to the fact that the use of normal analog-to-digital converters and a digital-to-analog converter following the signal processing circuit makes all the possible positive results of digital signal processing become quite illusory.